Tannin and lignin are natural polymers, which can be extracted from e.g. the bark of wood. For example the tannin content of Nordic softwood species, such as pine and spruce, is about 5-20%.
As tannin and lignin are natural biopolymers their use as components in glues instead of synthetic materials has been investigated in order to come up with a more environmentally friendly adhesive composition. Especially, the ability to replace synthetic phenol in final phenolic resins, such as phenol formaldehyde resin, has been the object of prior art.
Prior art discloses different types of adhesive compositions, such a phenolic glues, to be used with wood products. Examples of such glues include compositions comprising phenol formaldehyde resin. Traditionally synthetic phenol formaldehyde resins are produced by polymerizing phenol and formaldehyde in the presence of a catalyst. Examples of such catalysts are sodium hydroxide (NaOH) and acids. The method for producing phenol formaldehyde resin comprises adding formaldehyde in a stepwise manner to a phenol composition and thereafter rising the temperature of the formed composition up to 80-90° C. The composition is cooked at this temperature until a desired viscosity of the formed resin or polymer chain length is reached.
Prior art recognizes the use of lignin for the purpose of decreasing the amount of synthetic phenol in a resin composition. Lignin has previously been used for replacing phenol during the production of lignin-phenol-formaldehyde resin.
Also the use of tannin in adhesive compositions has previously been investigated. However, such production methods disclosed in prior art comprises only physical mixing of the tannin or its derivative into other components such as phenol formaldehyde resin to get a final glue composition.
Drawback of the prior art techniques, and especially of the tannin based production methods, is however that it has only been able to replace up to 30% of the synthetic phenol in the final resin, e.g. phenol formaldehyde resin, with tannin. Higher replacement results in unsatisfying properties of the produced glue.
The inventors have therefore recognized the need for a method, which would result in a higher phenol replacement in the composition and thus in a more environmentally friendly binder composition.